Identification of specific binding sequences is an integral part of research; delineation of antibody recognition sites, study of host-pathogen interaction, elucidation of signal transduction pathways, understanding of cellular responses, all these involve interaction of various macromolecules with one another; most of these interactions occurring between proteins or of a protein with another macromolecule.
For several years, expression libraries have been used for isolating ligands of desired specificity. However, the cumbersome technique of immobilizing expressed proteins on filters and screening large number of filters to obtain one specific binder is labour intensive and exhaustive, limiting the library size and number of clones that can be screened.
It was the concept of phage display introduced by Smith in 1985 (Smith, 1985), which revolutionized the field of proteomics. Large libraries came to be made in phage, and specific binders could be enriched from a milieu of millions under user-defined conditions. DNA encoding the selected molecule present inside the displaying phage particle could then be sequenced to know the identity of the binder. It was this beauty of the phage display system that led to an upsurge in the use of this technology in almost every field of science and unraveling of a plethora of applications, which are increasing everyday. Not limited to phage today, display of peptides/proteins on surface of bacteria, yeast and eukaryotic cells has also come into use.
The filamentous bacteriophage M13, with which was introduced the concept of display system is to date the most widely used system for display of peptides/proteins. Fusion to the minor coat protein, gIIIp and major coat protein, gVIIIp have beep used for display of a range of molecules of different sizes and structure (McCafferty et al., 1990; Scott and Smith, 1990; Kang et al., 1991). Simple biology, ease of culturing and isolating phages, small genome allowing easy manipulations, well documented protocols have all contributed to the immense popularity of the M13 system.
However, M13 morphogenesis occurs in the periplasm, therefore, it is essential that the molecules to be displayed be secretion competent. Though, versions of M13 which allow C-terminal display (Crameri and Blaser, 1996), have been developed, M13 continues to be used primarily as an N-terminal display system. Therefore, it is not useful for studying interactions involving free-C-terminus of display partner and for making full-length cDNA libraries from polyA mRNA.
One system, which obviates these problems associated with M13 display, is lambda display system. Phage lambda assembles in host cytosol and can be used for both N- and C-terminal display of molecules. There have been few papers in recent years that have described display of peptides/proteins on phage lambda as fusion to the capsid protein ‘d’ and tail protein ‘v’ (Dunn, 1995; Sternberg and Hoess, 1995; Mikawa et al., 1996). However, this system has not gained much popularity, which can be attributed to several reasons including the fact that    (i) the lambda phage biology is more complex than of M13 phage. Unlike M13 which grows by extruding from host cell, lambda can follow a lysogenic or lytic mode; therefore manipulation of lambda life cycle is more difficult,    (ii) lambda genome is very large (50 kb), therefore isolation of viral DNA, insertion of user defined restriction sites, cloning of foreign fragments and then packaging of the ligated product in vitro to make lambda particles is difficult and the library sizes achieved are less than those obtained with phage/phagemid based M13 vectors.